Carbon Dioxide Monitoring

Imagine you are in a meeting room in the afternoon. After 20 people are there for a while, the room gets stuffy, hot and generally uncomfortable. Sound familiar? It's because the meeting room has a constant-volume air supply, rather than a variable-volume supply that would increase ventilation when the room is occupied by lots of people. Sound simple? It is, and that's why most green building projects use carbon dioxide monitors to regulate ventilation levels in workspaces, with feedback to the building automation system that controls the fans and fresh-air intake.

In most buildings, carbon dioxide levels build up during the day, causing us to become sleepy after lunch in many cases. This hampers productivity and reduces comfort. It's like being on a warm, humid tropical beach, quite enjoyable during a vacation but not really conducive to regular office tasks. Typically, discomfort is triggered by carbon dioxide levels that are a few hundred parts per million above naturally occurring levels in the atmosphere.

Carbon dioxide monitors work on a simple precept: we breathe out carbon dioxide and take in oxygen; by monitoring carbon dioxide levels and using known levels of ventilation, we indirectly know how many people are in a room. Then we can adjust ventilation so that there are always a certain number of cubic feet per minute of fresh air per person. The monitors are relatively cheap and should be used in every project. The only issue is for engineers to specify good-quality monitors that hold their calibrations for several years, so that the ventilation levels remain appropriate for occupancy levels. The calibration needs to reference the baseline levels of carbon dioxide and to set limits for indoor air, so that there is always sufficient ventilation to keep the building air fresh.

The LEED standard for new construction handles this issue by requiring carbon dioxide monitors for densely occupied spaces (less than 40 square feet per person) and direct outdoor airflow measurements for regular office spaces, to achieve at least design-minimum ventilation. For naturally ventilated spaces, LEED requires carbon dioxide monitors between three and six feet above the floor.

Since most naturally ventilated spaces are typically "mixed mode" with some fan assist for stagnant air days, the monitors can be used to trigger the fans to pull outside air into the building. In buildings with no fans but with operable windows, a good green building design would have an alarm or signal to tell people to open the windows to let in more outside air.

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